This application is based on French Patent Application No. 00 03388 filed Mar. 16, 2000, the entirety of which application is hereby expressly incorporated by reference.
The present invention relates to a process for treating a gas by means of an active packing, of the type in which a cycle is implemented that comprises at least one treatment phase, in the course of which the gas to be treated is caused to circulate through the packing, and at least one phase for regenerating the packing. Other phases of such a cycle may be the pressurization or depressurization of the active packing.
The invention is applied in particular to processes for treating gaseous mixtures by selective adsorption, such as drying/CO2 removal of atmospheric air intended to be distilled, which may be a TSA process (Temperature Swing Adsorption or modulated temperature adsorption) or a PSA process (Pressure Swing Adsorption or modulated pressure adsorption); or the separation of gaseous mixtures by PSA or VPSA adsorption (Vacuum Pressure Swing Adsorption or vacuum modulated pressure adsorption), for example for producing oxygen from atmospheric air.
The invention especially concerns adsorption processes using at least one annular bed of adsorbent, and reference will be made to this application herein.
In the processes, the bed is contained in a space delimited between two concentric cylindrical grids. The gas to be treated passes through the annular bed and in a generally radial manner in a first direction, whereas a regeneration gas traverses the bed in a generally radial manner in the opposite direction.
To assure that the gas flows effectively traverse the totality of the adsorbent, it is necessary to provide at the top of the bed a device for preventing any by-pass or short-circuit. This may take the form for example of an annular guard of adsorbent, which may be delimited by two solid cylindrical walls connected in a sealed manner to the upper dome of the adsorption receptacle. This guard should be of enough material to remain between the two solid walls after packing of the adsorbent, which is produced after a certain number of cycles of operation owing to periodic dilation/contraction phenomena of the grids, of thermal origin.
Nevertheless, the guard constitutes a space in which the gaseous circulation is not controlled. In this space, certain zones may be poorly regenerated in PSA processes, either because part of the heat of the regeneration gas heats an adjacent wall and does not serve to regenerate the adsorbent, or, more fundamentally, because the heat of regeneration is going to be lost in regions wherein there is an over-abundance of adsorbent, or also because the total regeneration flow rate is insufficient to assure a good regeneration.
According to the configuration of the adsorbent bed, other edge effects may be produced, which is to say that other marginal regions of this bed may be formed in a manner unfavorable with respect to flow of at least one of the circulating fluids, which leads to using an excessive mass of adsorbent.
The document U.S. Pat. No. 4,541,851 describes a process of the type described above. In an effort to limit the edge effects and the local by-pass, the document EP-A-0 526 343 proposes to impede the free circulation of the gas in the upper region of the bed by using a flexible sealed membrane. Such a system is also described in the document U.S. Pat. No. 5,759,242.
The document EP A-0 778 082 proposes another solution consisting of one or several deflectors, so as to retard the treated gas during the adsorption phase. However, the deflectors also retard the regeneration gas, such that the problem of the existence of poorly regenerated regions remains.
The invention has as an object to provide a technique that efficiently and economically combats the edge effects and local by-passes, and to overcome the shortcomings observed for present adsorbers.
To that end, the invention has as an object a treatment process of the type described above, characterized in that, during at least one of the phases of the cycle, the gas outflow is locally increased or decreased in at least one marginal region of the packing.
The invention also has as an object a reactor having regenerable active packing intended to implement the process defined above. This reactor is characterized in that at least one marginal region of the packing is provided with means for establishing at least temporary communication with a point which lies at a different pressure from that of an adjacent region of the packing.